For seeking out potential phosphorus (P) sources, P fractions of biofilm sludge and P release, transformation and modeling in biofilm sludge treatment related to pH were investigated in this study. A laboratory-scale biofilm system was continuously operated to obtain the biofilm sludge. P release batch experiment was carried out by fermentation at different pH. P fraction and transformation were investigated with sequential extraction scheme and P-31 nuclear magnetic resonance spectroscopy. P release performance model was constructed based on extreme gradient boosting (XGBoost) method. Results showed that inorganic P-solid (IPsolid) occupied most in solid phase of raw biofilm sludge (69.21% total P-solid) and the content of releasable and bioavailable P (part of IP solid and organic P-solid) was 15.56 mg P/g TSS (90.57% of total P-solid), suggesting that biofilm sludge could be a potential P source. The optimum conditions for P release in biofilm sludge treatment process were pH = 5 and pH = 10, respectively, under acidic and alkaline conditions, and the corresponding maximum concentrations of orthophosphate-P were 72.44 mg/L and 76.84 mg/L. Moreover, pH played an important role on P transformation and redistribution in the treatment process. Credible accuracy of P release performance modeling based on XGBoost method was evaluated by root mean square error, mean absolute error and mean relative error, suggesting the feasibility of XGBoost method in modeling P release process, which was beneficial for the application of P release and recovery from biofilm sludge in practical wastewater treatment process.